The present invention relates to a hydraulic actuator assembly for the mold clamping unit of an injection molding machine. The actuator assembly provides a rapid-travel drive for mold opening and closing movements. It also provides a source of elevated mold clamping pressure.
In a hydraulic actuator assembly of the present type, there is a power cylinder, which is arranged in the axis of the mold clamping unit for building up the locking pressure for the injection mold. The cylinder spaces of the power cylinder are bounded by the power piston to create a high pressure space and a low pressure space. These two cylinder spaces are interconnected via overflow passages of the power piston, which overflow passages can be shut off by means of a hydraulically operable valve. A hydraulic drive cylinder with a stationary ram effects rapid closing of the injection mold. The actual cylinder of the hydraulic cylinder is formed by the piston rod of the hydraulic power cylinder. The actuator assembly has two single-acting travel cylinders arranged diametrically with respect to the hydraulic power cylinder. The travel cylinders have pistons for receiving and/or delivering hydraulic oil coming from the power cylinder or to be delivered to the same. The low pressure space of the power cylinder is in open connection with the cylinder spaces of the travel cylinders. The pistons of the travel cylinders and the power piston are each connected via the associated piston rods to the mold platen of the mold clamping unit. The actuator assembly has a control valve controlling the opening movement and the closing movement of the drive cylinder via a position detector in the form of a linear displacement voltage transformer in accordance with a quantity program, which quantity program is overridden by a pressure control via a pressure sensor.
A "quantity program" in the above sense controls the speeds of the drive cylinder on the basis of the actual values determined by the linear displacement voltage transformer.
A pressure program, overriding this speed control in the sense of the context of the present invention, has the effect that, when the admissible pressure envisaged by the program has been reached, the pressure program is automatically executed, which takes place on the basis of the actual values supplied by the associated pressure sensor.
"Control valve" in the sense of the context of the present invention is to be understood as a valve with a sliding valve member, which is known per se. The control valve is capable, together with a linear displacement voltage transformer and a pressure sensor, of developing a controlled pressure on the feed side and also on the delivery side of the control valve.
In the case of a known hydraulic system of this type according to U.S. Pat. No. 4,380,427, corresponding to West German Patent Specification No. 3,044,137, which patents are hereby incorporated herein by reference, the prerequisites exist for an extremely short yet nevertheless powerful mold clamping unit, with which it is not necessary to direct the entire displaced hydraulic oil to the reservoir during every opening or closing stroke of the mold clamping unit. The transition from the closing period of the mold clamping unit to its locking period is also not problematic to the extent that the closing stroke is exclusively effected by a clamping unit which is blocked off hydraulically from the power cylinder and the clamping force of the drive cylinder continues to act unchanged during the locking period.
It is also known from U.S. Pat. No. 4,565,116, corresponding to West German Patent Specification No. 3,238,111, which patents are hereby incorporated herein by reference, to control the flow cross-section of the connecting passages in a hydraulic system of the general type being considered, between the low pressure space of the power cylinder and the cylinder spaces of the travel cylinders, by an electrically adjustable throttle valve in dependence on a predetermined operating program. By such a further development, heavy mold clamping units can also be operated at high speed without any malfunctions. On the other hand, even in cases where low travelling speed is needed, a smooth, i.e. surge-free, running of the mold platen is ensured. In addition, it is possible to use the mold clamping unit position-independently. The speed sequence of the closing and opening movements and the time allocation for the braking phases can run according to a program. An injection mold which is difficult to open can be opened with increased force, without an additional ram-cylinder unit being necessary.
The throttle valve exhibiting these advantages and its installation in the mold clamping unit are, however, relatively costly, especially as certain connecting passages to be controlled must lead via the valve arranged outside the power cylinder. For this, passage bores have to be incorporated in the assembly block bearing the hydraulic system with the aid of cores during casting of said block. It has been found that such construction processes are difficult to master.
For further information with respect to these background aspects of the present invention, particularly the quantity program, pressure program, electromagnetically operated control valve, and voltage converter, see Canadian Patent No. 1,177,757, which is hereby incorporated herein by reference.